Use `pkg-config` to detect svn and apr libraries

[nega0]

Adds Qt's pkg-config support to src/src.pro for detecting svn and apr libraries. Also includes build examples in the README using the existing style.

pros:

• simpler config. less hard-coded paths.
• macOS support

cons:

• ugh. pkg-config. it want's to link all the things. requires additional dev/devel packages (eg liblz4-dev)
• macOS support assumes Homebrew

Dealing with pkg-config's need to link all the things could be dealt with by using pkg-config directly but this would greatly complicate src/src.pro.

closes #135